Jim, K&J Magnetics . They have every size shape and strength magnet you could ever want. I bought some from them, and am using one in my hump yard, to uncouple everything! Works great.
+1. This is the place I use. They specialize is neodymium magnets which are considerably stronger for their size than any other material. Consider that when making your selection. A pair of magnets a square inch or larger could probably lift your body weight.